1. Field of the Invention
This invention relates to presssure fixing device for pressure-fixing toner images on recording paper in an image forming apparatus such as a copying apparatus, a facsimile apparatus or a printer, and to a pressure fixing roller used therein.
2. Related Background Art
A heat fixing devicde using a heating roller has heretofore been used as a fixing device for fixing toner images on recording paper in a copying apparatus, a facsimile apparatus, a printer or the like, but this device has suffered from the disadvantages that it consumes a great deal of heat and accordingly a great deal of electric power, that the necessity of effecting temperature control complicates the device, that generation of a high temperature makes the device dangerous to handle, that a long waiting time is needed before the heat roller reaches a predetermined temperature and that much energy is wasted.
As a device which eliminates these disadvantages, there is known a pressure fixing device in which a pair of rollers are brought into contact with each other with high pressure and recording paper is passed through the portion of contact between the rollers thereby to fix toner images on the recording paper. In the conventional pressure fixing device, however, it is necessary to increase the rigidity of the rollers in order to provide a uniform pressure distribution between the rollers, and this in turn leads to the necessity of employing a material of great rigidity (Young's modulus) such as steel and making the diameter of the rollers large, and as a result, the device has unavoidably become bulky and heavy.
In order to make the rollers light in weight, the inventors have thought of using a ceramic material as the material of the rollers. From the viewpoint that the specific gravity of steel is 7.8-7.9, whereas the specific gravity of a ceramic material is considerably lower, the device can be made much lighter in weight. However, it has been found that the fragility peculiar to the ceramic material makes it difficult for practical use as the material of the pressure fixing roller.